Fabry-Perot photoelectric spectrometers have become widely employed in very-high-resolution studies of (extended) faint sources since this type of spectrometer has been found to have the highest luminosity-resolution product of any device that depends on the interference of light for its operation and has a spectrum as its output.
Two versions of the Fabry-Perot spectrometer exist, namely, the plane, or classical, Fabry-Perot spectrometer (FPP) and the spherical Fabry-Perot spectrometer (FPS). The spherical Fabry-Perot spectrometer has a larger throughput at very high resolution than can be obtained with the plane Fabry-Perot with practical size mirrors (i.e., useful diameters of &lt;150 mm); however, the spherical Fabry-Perot spectrometer has the inherent drawback that once the resolution (i.e., the spacing) is chosen, it cannot be changed except by obtaining a new pair of mirrors. Because the spacing, and therefore the resolution, of a plane Fabry-Perot spectrometer can be changed at will, this instrument is more versatile and this accounts for its wide usage, as opposed to the spherical version (which is normally used only in restricted applications).
The single-aperture plane Fabry-Perot spectrometer is sometimes not sufficiently luminous, however, to obtain measurements of faint sources with (arbitrary) time resolutions, and several schemes have heretofore been devised to increase the luminosity of the instrument. Increased luminosity has been attempted, for example, by replacement of the single aperture with a set of transparent annuli (to thus accept more than one fringe), using insect-eye lenses, measuring a number of elements at the same time using multiplexing techniques, multiple detectors, and/or segmented single detectors that include TV camera tubes.
The increase in luminosity heretofore obtained, however, has been modest, due at least in part, because the precision to which multiple annuli masks can be made limits the number of these annuli in a practical mask, the annuli scheme is useful only very near a fixed wavelength, multiple and segmented detector approaches reach a practical limit after about ten detector/segments due of mechanical and calibration problems, and/or the TV tube approach suffers from limited sensitivity and dynamic range compared with other detectors, such as photomultiplier tubes, for example.
A Fabry-Perot spectrometer is described in my article entitled "Measurement of thermospheric temperatures and winds by remote Fabry-Perot spectrometry", Optical Engineering, Volume 19, No. 4, July/August, 1980, pages 518-532. In addition, the Fabry-Perot interferometer is shown and described, for example, in U.S. Pat. Nos. 4,225,236 and 3,984,190.
The twin-etalon scanning spectrometer, as used in this invention, is described in "TESS: a high-luminosity high-resolution twin-etalon scanning spectrometer", Applied Optics, Volume 20, No. 21, Nov. 1, 1981, pages 3687-3688, and "Analytical description of a Fabry-Perot spectrometer. 7: TESS, a high-luminosity high-resolution twin-etalon scanning spectrometer", Applied Optics, Volume 21, No. 3, Feb. 1, 1982, pages 507-512.